Enzymes are used as industrial catalysts for the large and laboratory scale economical production of fine and specialty chemicals (Jones, J. B., Tetrahedron 42: 3351-3403 (1986)), for the production of foodstuffs (Zaks et. al., Trends in Biotechnology 6: 272-275 (1988)), and as tools for the synthesis of organic compounds (Wong, C.-H., Science 244: 1145-1152 (1989); CHEMTRACTS-Org. Chem. 3: 91-111 (1990); Klibanov, A. M., Acc. Chem. Res. 23: 114-120 (1990)).
Enzyme-based manufacturing can significantly reduce the environmental pollution burden implicit in the large scale manufacturing of otherwise unusable chemical intermediates, as shown in the large scale production of acrylamide using the enzyme, nitrile hydratase (Nagasawa, T. and Yamada, H., Trends in Biotechnology 7: 153-158 (1989)).
Enzymes are also used in biosensor applications to detect various substances of clinical, industrial and other interest (Hall, E., "Biosensors", Open University Press (1990)). In the clinical area, enzymes may be used in extracorporeal therapy, such as hemodialysis and hemofiltration, where the enzymes selectively remove waste and toxic materials from blood (Klein, M. and Langer, R., Trends in Biotechnology 4: 179-185 (1986)). Enzymes are used in these areas because they function efficiently as catalysts for a broad range of reaction types, at modest temperatures, and with substrate specificity and stereoselectivity. Nonetheless, there are disadvantages associated with the use of soluble enzyme catalysts which have limited their use in industrial and laboratory chemical processes (Akiyama et. al., CHEMTECH 627-634 (1988)).
Enzymes are expensive and relatively unstable compared to most industrial and laboratory catalysts, even when they are used in aqueous media where enzymes normally function. Many of the more economically interesting chemical reactions carried out in common practice are incompatible with aqueous media, where, for example, substrates and products are often insoluble or unstable, and where hydrolysis can compete significantly. In addition, the recovery of soluble enzyme catalyst from product and unreacted substrate in the feedstock often requires the application of complicated and expensive separation technology. Finally, enzymes are difficult to store in a manner that retains their activity and functional integrity, for commercially reasonable periods of time (months to years) without having to resort to refrigeration (4.degree. C. to -80.degree. C. to liquid N.sub.2 temperatures), or to maintenance in aqueous solvents of suitable ionic strength, pH, etc.
Enzyme immobilization methods have, in many instances, circumvented these disadvantages. Immobilization can improve the stability of enzyme catalysts and protect their functional integrity in the harsh solvent environments and extreme temperatures characteristic of industrial and laboratory chemical processes (Hartmeier, W., Trends in Biotechnology 3: 149-153 (1985)). Continuous flow processes may be operated with immobilized enzyme particles in columns, for example, where the soluble feedstock passes over the particles and is gradually converted into product. As used herein, the term enzyme immobilization refers to the insolubilization of enzyme catalyst by attachment to, encapsulation of, or by aggregation into macroscopic (10.sup.-1 mm) particles.
A number of useful reviews of enzyme immobilization methods have appeared in the literature (Maugh, T. H., Science 223: 474-476 (1984); Tramper, J., Trends in Biotechnology 3: 45-50 (1985)). Maugh describes five general approaches to the immobilization of enzymes. These include: adsorption on solid supports (such as ion-exchange resins); covalent attachments to supports (such as ion-exchange resins, porous ceramics or glass beads); entrapment in polymeric gels; encapsulation; and the precipitation of soluble proteins by cross-linking them with bifunctional reagents in a random and undefined manner. In addition, one can immobilize whole cells (usually dead and made permeable) which have expressed the desired enzyme activity at high levels (e.g., Nagasawa, T. and Yamada, H., Trends in Biotechnology 7: 153-158 (1989)).
Each of these immobilization procedures has its own advantages and limitations and none can be considered optimal or dominating. In most of them, the enzyme catalyst ultimately represents only a small fraction of the total volume of material present in the chemical reactor. As such, the bulk of the immobilized medium is made up of inert, but often costly carrier material. In all of them, the immobilizing interactions of the enzyme catalyst molecules with each other and/or with the carrier material tend to be random and undefined. As a result, although these interactions confer some enhanced stability to the enzyme catalyst molecules, their relative non-specificity and irregularity makes that stabilization sub-optimal and irregular. In most cases, access to the active site of the enzyme catalyst remains ill-defined. In addition, the immobilization methods described above fail to deal with problems associated with storage and refrigeration. Nor can conventionally immobilized enzymes generally be manipulated, as in being exchanged into one or another solvent of choice, without risk to the structural and functional integrity of the enzyme. In practical terms, except for the attached tether to the carrier particle, conventionally immobilized enzymes bear close resemblance to soluble enzymes, and share with them a susceptibility to denaturation and loss of function in harsh environments. In general, immobilization methods lead to a reduction of observed enzyme-catalyzed reaction rates relative to those obtained in solution. This is mostly a consequence of the limits of inward diffusion of substrate and outward diffusion of product within the immobilized enzyme particle (Quiocho, F. A., and Richards, F. M., Biochemistry 5: 4062-4076 (1967)). The necessary presence of inert carrier in the immobilized enzyme particles increases the mean free path between the solvent exterior of the immobilized enzyme particle and the active site of the enzyme catalyst and thus exacerbates these diffusion problems. When dealing with immobilized cells, the diffusion problem is particularly severe, even if cell walls and membranes are made permeable to substrate and product in some way. One would further be concerned with the multitude of contaminating enzymatic activities, metabolites, and toxins contained in cells, and with the stability of cells in harsh solvents or extreme temperature operating environments. An improved immobilization technique which avoids the limitations of the presently available methods would be helpful in promoting the use of enzymes as industrial catalysts, particularly if it were shown to be useful on a large scale (Daniels, M. J., Methods in Enzymology 136: 371-379 (1987)).